Infrared (IR) detection and imaging systems are becoming increasingly important in many different applications including civil, industrial, automotive, medical, astronomical, law enforcement and public safety sectors. Generally IR radiation can be detected by photon or thermal detectors.
Thermal detectors absorb IR radiation which induces a change in the device temperature; this effect can be measured through a change in a physical property of the detector such as resistance or polarization for example. The main advantage of thermal infrared detectors lies in the fact that they can operate at room temperature and hence avoid the cost of cryogenic cooling. However, thermal detectors suffer from a number of disadvantages which limit their performance including slow response, low sensitivity, necessity for vacuum packaging and thermal insulation.
In a photon detector, incident IR photons get absorbed by carriers (electrons or holes) enabling them to make a transition from a non-conducting band to a conducting band within the device structure. The output signal can then be measured as photocurrent or a photovoltage across the detector. The most widely used IR photodetector is based on the HgCdTe ternary semiconductor alloy, which is characterized with high quantum efficiency (>70%) and wide band spectral sensitivity. However, as with most IR photodetector HgCdTe based detectors cannot operate at room temperature and rely on cryogenic cooling for their operation.
A recently introduced IR photodetector known as Quantum Well Infrared Photodetector (QWIP), has attracted a lot of attention due to its superior performance offering fast response, large sensitivity and multi-spectral response capability. QWIP technology is based on the well developed III-V material system with a large industrial base offering high uniformity and yield due to the mature micro-fabrication technology. Therefore, two-dimensional (2D) QWIP based Focal Plane Arrays (FPAs) are much easier and less expensive to fabricate than HgCdTe based FPAs. Furthermore, in the long wavelength and very long wavelength IR applications, HgCdTe suffers from serious problems due to non-uniformity and low process yield. However, the main disadvantage of QWIP technology is the requirement of cooling to cryogenic temperatures typically below 80K.